From: Peter van Dijk <peter.van.dijk@powerdns.com>
Date: Thu, 15 Apr 2021 12:21:44 +0000 (+0200)
Subject: start off recursor FAQ with some talk about EDNS bufsizes
X-Git-Tag: dnsdist-1.6.0-rc1~4^2~7
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=bbbc212d01fdd76bb6d602b922b9d9aa8b4fb9c3;p=thirdparty%2Fpdns.git

start off recursor FAQ with some talk about EDNS bufsizes
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diff --git a/pdns/recursordist/docs/appendices/FAQ.rst b/pdns/recursordist/docs/appendices/FAQ.rst
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+Frequently Asked Questions
+==========================
+
+This document lists categorized answers and questions with links to the relevant documentation.
+
+EDNS bufsize in response packets
+--------------------------------
+
+You may have spotted the 512 in something like the following (after ``EDNS ... udp:``)::
+
+  $ dig example.com @9.9.9.9
+  
+  ; <<>> DiG 9.11.5-P4-5.1+deb10u3-Debian <<>> example.com @9.9.9.9
+  ;; global options: +cmd
+  ;; Got answer:
+  ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 20155
+  ;; flags: qr rd ra ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
+  
+  ;; OPT PSEUDOSECTION:
+  ; EDNS: version: 0, flags:; udp: 512
+  ;; QUESTION SECTION:
+  ;example.com.           IN  A
+  
+  ;; ANSWER SECTION:
+  example.com.        43200   IN  A   93.184.216.34
+  
+  ;; Query time: 86 msec
+  ;; SERVER: 9.9.9.9#53(9.9.9.9)
+  ;; WHEN: Thu Apr 15 13:56:34 CEST 2021
+  ;; MSG SIZE  rcvd: 56
+
+and wonder 'why is the Recursor using a bufsize of 512? Did we not decide on a Flag Day, all together, that we would use 1232?'
+
+The EDNS buffer size in a DNS packet, generated by side A, tells the recipient of that packet (side B) the maximum packet size that side A will accept from side B.
+So, when the Recursor talks to an Authoritative, the Recursor reports the buffer size the Authoritative is allowed to use to it - usually 1232 (:ref:`setting-edns-outgoing-bufsize`).
+But the example above is the Recursor responding to a client, and it is telling the client 'from you, I accept packets of up to 512 bytes'.
+Or, to say it differently, the Recursor is telling the client that *questions* must fit in 512 bytes.
+
+Similarly, the maximum size of a response from the Recursor to a client is governed by the buffer size reported by the client, and the :ref:`setting-udp-truncation-threshold` setting in the Recursor configuration.
+
+To see the buffer size the Recursor is reporting to authoritatives, ask an authoritative::
+
+  $ dig txt header.lua.powerdns.org +short @9.9.9.9
+  "id: 52938, aa: false, rd: false, ad: false, cd: false, do: true, ednsbufsiz: 1232, tcp: false"